Line a muffin tin with cupcake liners and preheat the oven to 150C or 350F.
Combine the flour, baking powder and salt in a medium bowl. Sieve the cocoa into the flour mixture and stir everything with a whisk until well combined.
Whisk the sugar, egg, oil, milk and vanilla extract together until well combined.
Add the wet ingredients into the dry and mix until no flour lumps remain.
While continuously whisking, add in the hot water.
Pour into your prepared liners and bake for 28-30 minutes, until a toothpick inserted in the center comes out clean.
Let the cupcakes cool completely before frosting.
With a hand mixer, beat the butter with a pinch of salt until fluffy, for about 1 minute. Sieve in the powdered sugar and cocoa powder and continue beating until smooth.
Add in the Nutella and mascarpone and continue mixing until well combined.
Fill into a piping bag fitted with Wilton 1M tip and pipe onto the cupcakes. Optional: Decorate with roasted hazelnut sprinkles or decorations of choice.